package com.baomidou.kisso.common.bcprov.math.ec;

import java.math.BigInteger;

/**
 * Class implementing the WNAF (Window Non-Adjacent Form) multiplication
 * algorithm.
 */
public class WNafL2RMultiplier extends AbstractECMultiplier {

	/**
	 * Multiplies <code>this</code> by an integer <code>k</code> using the
	 * Window NAF method.
	 * @param k The integer by which <code>this</code> is multiplied.
	 * @return A new <code>ECPoint</code> which equals <code>this</code>
	 * multiplied by <code>k</code>.
	 */
	protected ECPoint multiplyPositive( ECPoint p, BigInteger k ) {
		// Clamp the window width in the range [2, 16]
		int width = Math.max(2, Math.min(16, getWindowSize(k.bitLength())));

		WNafPreCompInfo wnafPreCompInfo = WNafUtil.precompute(p, width, true);
		ECPoint[] preComp = wnafPreCompInfo.getPreComp();
		ECPoint[] preCompNeg = wnafPreCompInfo.getPreCompNeg();

		int[] wnaf = WNafUtil.generateCompactWindowNaf(width, k);

		ECPoint R = p.getCurve().getInfinity();

		int i = wnaf.length;

		/*
		 * NOTE This code optimizes the first window using the precomputed points to substitute an
		 * addition for 2 or more doublings.
		 */
		if ( i > 1 ) {
			int wi = wnaf[--i];
			int digit = wi >> 16, zeroes = wi & 0xFFFF;

			int n = Math.abs(digit);
			ECPoint[] table = digit < 0 ? preCompNeg : preComp;

			/*
			 * NOTE: We use this optimization conservatively, since some coordinate systems have
			 * significantly cheaper doubling relative to addition.
			 * 
			 * (n << 2) selects precomputed values in the lower half of the table
			 * (n << 3) selects precomputed values in the lower quarter of the table
			 */
			//if ((n << 2) < (1 << width))
			if ( (n << 3) < (1 << width) ) {
				int highest = LongArray.bitLengths[n];
				int lowBits = n ^ (1 << (highest - 1));
				int scale = width - highest;

				int i1 = ((1 << (width - 1)) - 1);
				int i2 = (lowBits << scale) + 1;
				R = table[i1 >>> 1].add(table[i2 >>> 1]);

				zeroes -= scale;

				//              System.out.println("Optimized: 2^" + scale + " * " + n + " = " + i1 + " + " + i2);
			} else {
				R = table[n >>> 1];
			}

			R = R.timesPow2(zeroes);
		}

		while ( i > 0 ) {
			int wi = wnaf[--i];
			int digit = wi >> 16, zeroes = wi & 0xFFFF;

			int n = Math.abs(digit);
			ECPoint[] table = digit < 0 ? preCompNeg : preComp;
			ECPoint r = table[n >>> 1];

			R = R.twicePlus(r);
			R = R.timesPow2(zeroes);
		}

		return R;
	}


	/**
	 * Determine window width to use for a scalar multiplication of the given size.
	 * 
	 * @param bits the bit-length of the scalar to multiply by
	 * @return the window size to use
	 */
	protected int getWindowSize( int bits ) {
		return WNafUtil.getWindowSize(bits);
	}
}
